Pole-piece unit, method for assembling the same, and magnetic field generator

ABSTRACT

A magnetic field generator comprises a pair of pole-piece units. The pair of pole-piece units respectively include plate yokes. Each of the plate yokes includes a surface facing the other&#39;s and provided with a permanent magnet group and a pole piece. The pole piece includes an annular projection having a plurality of annular-projection pieces and a permanent magnet for reduction of magnetic flux leakage installed on an outside surface of each annular-projection piece. When assembling the pole piece, first, the permanent magnet for reduction of magnetic flux leakage is fixed on the outside surface of each annular-projection piece. At this time, the permanent magnet is slid on the flat outside surface of the annular-projection piece, to a desired position on the annular-projection piece, and then fixed. Each of the annular-projection pieces mounted with the permanent magnet is fixed on a base plate. At this time, the annular-projection piece is lowered on a guide stake attached on the base plate, allowing the stake into a guide hole formed in the annular-projection piece, thereby guiding the annular-projection piece onto the base plate. The pair of pole-piece units are magnetically connected by a column yoke. The permanent magnet for reduction of magnetic flux leakage may be disposed with a magnetizing direction thereof being slanted with respect to a main surface of the permanent magnet group.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a pole-piece unit, a method forassembling the same, and a magnetic field generator. More specifically,the present invention relates to a pole-piece unit, a method forassembling the same, and a magnetic field generator used for an MRIapparatus and so on which requires an intense magnetic field greaterthan 0.3 T.

[0003] 2. Description of the Related Art

[0004] In this kind of large magnetic field generator, e.g. a magneticfield generator for an MRI, in order to reduce magnetic flux leakagefrom pole pieces and to concentrate the magnetic flux effectively onto aspace between the pole pieces, the applicant of the present inventionproposed an arrangement, as disclosed in the Japanese Utility Model (ofexamined Application for opposition) No. 2-49683, in which a permanentmagnet for repelling the magnetic flux leakage is provided on an outsidesurface of an annular projection.

[0005] When an intense magnetic field is required in this related art, aR—Fe—B magnet which has an intense magnetic power is used as thepermanent magnet. In this case, the permanent magnet to be bonded comesunder an intense repelling force when brought near the permanent magnetwhich is already bonded. Thus, it is difficult to bond the permanentmagnet to the annular projection which is formed as a single piece,resulting in a poor production efficiency.

[0006] Further, according to the above related art, the permanent magnetfor repelling the magnetic flux leakage is a magnet magnetized in adirection in parallel to a main surface of a permanent magnet used forgeneration of the magnetic field. If a uniform space is to be enlarged,the amount of magnet must be increased, causing a problem of increasedcost.

SUMMARY OF THE INVENTION

[0007] It is therefore a primary object of the present invention toprovide a pole-piece unit, a method for assembling the same, and amagnetic field generator for improved production efficiency.

[0008] Another object of the present invention is to provide a magneticfield generator in which the uniform space can be increased withoutincreasing the cost.

[0009] According to an aspect of the present invention, there isprovided a pole-piece unit comprising: a plate yoke; a permanent magnetgroup provided on a main surface of the plate yoke; and a pole pieceprovided on a main surface of the permanent magnet group; wherein thepole piece includes an annular projection having a plurality ofannular-projection pieces, and a permanent magnet for reduction ofmagnetic flux leakage provided on an outside surface of each of theannular-projection pieces.

[0010] According to another aspect of the present invention, there isprovided a method for assembling a pole-piece unit, comprising: a firststep of fixing a permanent magnet onto an outside surface of anannular-projection piece; and a second step of fixing theannular-projection piece already mounted with the permanent magnet ontoa main surface of a base plate.

[0011] According to this invention, the permanent magnet is fixed inadvance onto the outside surface of each annular-projection piece, and apredetermined number of the annular-projection pieces each mounted withthe permanent magnet are installed onto the base plate. Therefore, thereis no need for installing the permanent magnets to the annularprojection which has been disposed on the base plate. Thus, fixation ofthe permanent magnets becomes easy, and assembling of the pole-pieceunit becomes easy, making possible to improve productivity inmanufacture of the pole-piece unit.

[0012] According to another aspect of the present invention, there isprovided a magnetic field generator comprising a pair of the pole-pieceunits, and a column yoke magnetically connecting the pair of thepole-piece units.

[0013] According to this invention, by using the above pole-piece unitwhich can be assembled easily, the magnetic field generator is obtainedeasily, and productivity is increased.

[0014] Preferably, in the above method for assembling the pole-pieceunit, the annular-projection piece has a guide hole; and the second stepincludes a step A of attaching a guide stake to the base plate, and astep B of guiding the annular-projection piece already mounted with thepermanent magnet to the main surface of the base plate, by inserting theguide stake into the guide hole. In this case, the annular-projectionpiece can be guided to a desired place on the base plate, and therefore,positioning of the annular-projection piece becomes easy and theassembling of the pole-piece unit becomes easy.

[0015] According to still another aspect of the present invention, thereis provided a method for assembling a pole-piece unit including anannular-projection piece having a flat outside surface, wherein apermanent magnet is slid on the outside surface for fixation of thepermanent magnet to a desired position on the annular-projection piece.

[0016] According to this invention, since the outside surface of theannular-projection piece is a flat surface, the permanent magnet can beslid linearly and easily on the outside surface. Therefore, thepermanent magnet can be easily transported and fixed to a desiredposition on the outside surface of the annular-projection piece. Thus,the assembling of the pole-piece unit becomes easy, and productivity inthe manufacture of the pole-piece unit is improved.

[0017] According to still another aspect of the present invention, thereis provided a magnetic field generator comprising: a plate yoke; apermanent magnet group provided on a main surface of the plate yoke; anda pole piece provided on a main surface of the permanent magnet group;wherein the pole piece includes an annular projection and a permanentmagnet for reduction of magnetic flux leakage provided on an outsidesurface of the annular projection, and the permanent magnet forreduction of magnetic flux leakage is disposed, with a magnetizingdirection thereof being slanted with respect to the main surface of thepermanent magnet group.

[0018] According to this invention, by disposing the permanent magnetfor reduction of magnetic flux leakage, with the magnetizing directionslanted with respect to the main surface of the permanent magnet group,greater amount of magnetic flux can be introduced into an imaging space,and the uniform space can be enlarged. Therefore, the uniform space ofmagnetic field strength can be enlarged without increasing the amount ofpermanent magnet to be used, and without increasing cost.

[0019] Preferably, the permanent magnet for reduction of magnetic fluxleakage is generally rectangular parallelepiped, and the magnetizingdirection makes an acute angle with respect to a main surface of thepermanent magnet. In this case, by only disposing the permanent magnetfor reduction of magnetic flux leakage directly onto the main surface ofthe permanent magnet group, disposition of the permanent magnet forreduction of magnetic flux leakage having a desired magnetizing anglecan be carried out easily.

[0020] Further, preferably, the permanent magnet for reduction ofmagnetic flux leakage is generally rectangular parallelepiped, and isslanted with respect to the main surface of the permanent magnet group.In this case, a permanent magnet magnetized in any direction can be usedas the permanent magnet for reduction of magnetic flux leakage.

[0021] Further, preferably, a dimension of the pole piece is determinedin accordance with a magnetizing angle of the permanent magnet forreduction of magnetic flux leakage. By adjusting the size of the polepiece in accordance with a magnetizing angle of the permanent magnet forreduction of magnetic flux leakage, a center magnetic field strength canbe increased without increasing the amount of permanent magnet to beused and without increasing cost. Especially, it is more preferable ifthe magnetizing angle of the permanent magnet for reduction of magneticflux leakage is greater than 0 degree and smaller than 60 degrees.

[0022] It should be noted here that in this specification, the term“magnetizing angle” means an angle made by a main surface of thepermanent magnet group and a direction in which the permanent magnet forreduction of magnetic flux leakage is magnetized. The term “uniformspace” means a magnetic-field space in which the magnetic fielduniformity is within 100 ppm. The term “center magnetic field strength”means a magnetic field strength at a center portion of the uniformspace.

[0023] The above objects, other objects, characteristics, aspects andadvantages of the present invention will become clearer from thefollowing description of embodiments to be presented with reference tothe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024]FIG. 1 is a perspective view showing an outline of an embodimentof the present invention;

[0025]FIG. 2 is an illustration showing a primary portion of apole-piece unit;

[0026]FIG. 3 is a plan view showing a pole piece;

[0027]FIG. 4 is a perspective view showing a permanent magnet;

[0028]FIG. 5A is a perspective view showing an annular-projection piecemounted with the permanent magnet,

[0029]FIG. 5B is a plan view thereof;

[0030]FIG. 6 is an illustration showing a step of installing thepermanent magnet to the annular-projection piece;

[0031]FIG. 7A is an illustration showing a step of installing anannular-projection piece on a base plate, FIG. 7B and FIG. 7C areillustrations showing an order of installing the annular-projectionpieces;

[0032]FIG. 8 is an illustration showing a method for installing the polepiece on a permanent magnet group;

[0033]FIG. 9 is an illustration showing an outline of another embodimentof the present invention;

[0034]FIG. 10 is an illustration showing an example of a pressingapparatus.

[0035]FIG. 11A is a graph showing relationships of a magnetizing anglewith a center magnetic field strength and with a diameter of a uniformspace, FIG. 11B is a graph showing a relationship between themagnetizing angle and the center magnetic field strength in a case wherethe diameter of the uniform space is kept constant;

[0036]FIG. 12 is a table showing an example of comparison in terms of atotal weight of magnets when the center magnetic field strength and thedistance between the pole pieces are kept constant; and

[0037]FIG. 13 is an illustration showing an outline of still anotherembodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0038] Now, embodiments of the present invention will be described withreference to the accompanying drawings.

[0039] Referring now to FIG. 1, a magnetic field generator 10 as a firstembodiment of the present invention, to be used for an MRI, is an opentype magnetic field generator comprising a pair of pole-piece units 11a, 11 b facing each other with a space in between. The pole-piece units11 a, 11 b respectively include plate yokes 12 a, 12 b. The plate yoke12 a includes a generally disc-shaped main body 14 a and two connectingportions 16 a, 18 a extending from the main body 14 a. Likewise, Theplate yoke 12 b includes a generally disc-shaped main body 14 b and twoconnecting portions 16 b, 18 b extending from the main body 14 b.

[0040] The main body 14 a of the plate yoke 12 a and the main body 14 bof the plate yoke 12 b have respective surfaces faced with each otherand provided with permanent magnet groups 20 a and 20 b. The permanentmagnet groups 20 a, 20 b have respective surfaces faced with each otherand provided with pole pieces 22 a and 22 b respectively fastenedthereto.

[0041] As shown in FIG. 2, the permanent magnet group 20 b includes aplurality of generally rectangular parallelepiped unit-piece magnets 24each made of a R—Fe—B magnet for example. The R—Fe—B magnet (wherein Rrepresents rare-earth elements including yttrium Y) is disclosed in theU.S. Pat. Nos. 4,770,723 or 4,792,368. The same applies to the permanentmagnet group 20 a.

[0042] The pole piece 22 b includes a base plate 26 made of disc-shapediron for example, disposed on the permanent magnet group 20 b. The baseplate 26 has an upper surface provided with a silicon steel plate 28 forpreventing generation of eddy current. The silicon steel plate 28 isfixed with an adhesive onto the base plate 26.

[0043] The base plate 26 has a circumferential portion formed with anannular projection 32, made of iron for example, for increasing magneticfield strength around the circumferential portion. As shown in FIG. 3,the annular projection 32 includes a plurality (e.g. eight, according tothe present embodiment) of annular-projection pieces 34. Each of theannular-projection pieces 34 has a concavely arced inside surface and aflat outside surface. The annular projection 32 is formed by boltingeach of the annular-projection pieces 34 with screws 36 to the baseplate 26 of a circumferential portion of the silicon steel plate 28. Inother words, the annular projection 32 is made of the radially dividedpieces. The annular projection 32, or more specifically each of theannular projection pieces 34, is formed with two guide holes 38 each asa vertical through hole for assembling. Each of the guide holes 38 has athreaded inner surface to be engaged by a screw 92 to be describedlater.

[0044] The outside surface of each annular-projection piece 34 ismounted with a permanent magnet 40 for reduction of magnetic fluxleakage. The permanent magnet 40 induces magnetic flux to concentratebetween the pole pieces 22 a, 22 b, making possible to reduce themagnetic flux leakage. As shown in FIG. 4, the permanent magnet 40includes a plurality of generally rectangular parallelepiped unit-piecemagnets 42 each made of a R—Fe—B magnet for example. According to thepresent embodiment, a total of sixteen (=2×2×4) unit-piece magnets 42are stacked into a generally rectangular parallelepiped permanent magnet40. It should be noted here that through holes 44 are formed by cuttingoff a corner portion of each unit-piece magnet 42.

[0045] As will be understood clearly from FIG. 5A and FIG. 5B, arectangular retainer plate 46, made of a nonmagnetic material such asSUS304 and aluminum, is disposed on an outside surface of each permanentmagnet 40 mounted to the outside surface of the annular-projection piece34. The permanent magnet 40 and the retainer plate 46 are fixed to theannular-projection piece 34 by fixing screws 48 inserted through thethrough holes 44. Further, as will be understood from FIG. 2 and FIG.5A, in order to reduce the magnetic flux leakage from a bottom portionof the permanent magnet 40, it is preferable that a lower portion of thepermanent magnet 40 is extended to make the bottom of the permanentmagnet 40 contact the permanent magnet group 20 b. The same applies tothe pole piece 22 a.

[0046] Returning to FIG. 1, a movable yoke 50 for fine adjustment of themagnetic field is disposed at a center portion of the plate yoke 12 a.The movable yoke 50 is provided with three bolts 52 for adjustingvertical position thereof, and three stoppers 54 for regulating theposition thereof. The same arrangement applies to the plate yoke 12 b.The connecting portions 16 a, 18 a of the plate yoke 12 a arerespectively provided with threaded holes 56 to which hoisting hooks areto be attached.

[0047] The plate yokes 12 a, 12 b as described above are magneticallyconnected by two cylindrical column yokes 58. The column yokes 58 arerespectively disposed between connecting portions 16 a, 16 b and betweenthe connecting portions 18 a, 18 b, and fixed to the respectiveconnecting portions 16 a, 18 a of the plate yoke 12 a by fixing bolts60. Further, the plate yoke 12 b has a lower surface provided with threeleg portions 62, respectively at a front portion and locationscorresponding to the two column yokes 58.

[0048] Next, steps of assembling the permanent magnet 40 and retainerplate 46 to the annular-projection piece 34 in the magnetic fieldgenerator 10 will be described with reference to FIG. 6.

[0049] First, the annular-projection piece 34 is placed, with theoutside surface thereof facing upward, and fixed by fixing screws 66 ona working table 64. The working table 64 is provided with a positioningjig 68, which is a plate-like piece of nonmagnetic material standing tocontact an end of the outside surface of the annular-projection piece34, i.e. the surface to which the permanent magnet is fixed. Thepositioning jig 68 is fixed to the working table 64 by screws (notillustrated) for example. On the other hand, at the other end of theoutside surface of the annular-projection piece 34, a sliding table 70is placed so that the outside surface of the annular-projection piece 34and an upper surface of the sliding table 70 become generally flush witheach other, providing a continuous plane.

[0050] With the above preparation, a first magnet block 72 is disposedon the sliding table 70. The magnet block 72 is a component formed bystacking and bonding eight (=2×2×2) of the unit-piece magnet 42 shown inFIG. 4. The magnet block 72 is held firmly by a holding portion 74 andis pushed by an arm 76 out of the sliding table 70 onto the outsidesurface of the annular-projection piece 34, sliding on theannular-projection piece 34 to contact the positioning jig 68.

[0051] During the above operation, since the annular-projection piece 34is made of soft iron, the magnet block 72 is strongly attracted to theannular-projection piece 34. While being attracted strongly, the firstmagnet block 72 is pushed up by a screw 78 threaded from beneath theworking table 64, and an adhesive is applied between the magnet block 72and the annular-projection piece 34. Thereafter, the screw 78 isunthreaded to allow the magnet block 72 to bond to theannular-projection piece 34. Then, likewise, a second magnet block 72 isheld firmly by the holding portion 74 and is pushed by the arm 76 towardthe outside surface of the annular-projection piece 34. The pushingoperation is stopped just before reaching a predetermined fixingposition, to allow application of the adhesive to the predeterminedfixing position on the annular-projection piece 34, and to an endsurface of the first magnet block 72. Thereafter, the second magnetblock 72 is pushed by the arm 76 to the predetermined fixing position tocomplete the fixing. It should be noted here that there is a repellingforce between the magnet block 72 which is already fixed and the magnetblock 72 which is to be fixed. If the magnet block 72 to be fixed isrepelled off the surface, an air cylinder (not illustrated) may beprovided above the holding portion 74 to press the magnet block 72downward. The magnet blocks 72, i.e. the permanent magnets 40 are thusfixed to the outside surface of the annular-projection piece 34. In theabove operation, since the outside surface of the annular-projectionpiece 34 is a flat surface, the magnet block 72 can be slid linearly andeasily on the outside surface. Therefore, the magnet block 72 can beeasily transported and fixed to a desired position on the outsidesurface of the annular-projection piece 34.

[0052] Thereafter, the retainer plate 46 is disposed on an upper surfaceof the magnet block 72 and then fastened by the fixing screws 48.

[0053] Next, steps for assembling the pole piece 22 b will be describedwith reference to FIG. 7A through FIG. 7C.

[0054] First, as shown in FIG. 7A, a guide stake 80 is threaded into thebase plate 26 of the pole piece 22 b. Next, the annular-projection piece34, with the permanent magnet 40 installed thereto, is lowered on theguide stake 80, allowing the guide stake 80 into the guide hole 38 ofthe annular-projection piece 34, thereby placing the annular-projectionpiece 34 onto the guide plate 26. If the annular-projection piece 34cannot be easily lowered due to repelling force from theannular-projection piece 34 which is already installed, then the guidestake 80 may be formed with a male thread in advance, and a mating nut(not illustrated) to be threaded by the guide stake 80 may be disposedon the annular-projection piece 34. With this arrangement, theannular-projection piece 34 can be lowered by tightening the nut. Thelowered annular-projection piece 34 is fixed with the screws 36 onto thebase plate 26. Thereafter, the guide stake 80 is removed.

[0055] In the above described installation of the annular-projectionpiece 34, annular-projection pieces 34 are first installed at everyother positions as shown in FIG. 7B, and then, as shown in FIG. 7C, therest of the annular-projection pieces 34 are installed, to fill thepositions between the annular-projection pieces 34 which are alreadyinstalled. During the installation, the above described method ofinstallation by tightening the nut is desirably used because therepelling force is very strong.

[0056] The pole piece 22 b assembled according to the above describedmethod, is then fixed onto the permanent magnet group 20 b by using alift 82, for example, as shown in FIG. 8. The lift 82 includes a liftingtable 84 made of a nonmagnetic material such as a stainless steel(SUS304). The lifting table 84 can be vertically moved by rotation ofthreaded driving shafts 88 built on a base table 86. The base table 86has an upper surface provided with a turntable 90, on which the plateyoke 12 b and the permanent magnet group 20 b are disposed. The polepiece 22 b is suspended by screws 92 which penetrate the lifting table84. The screws 92 are threaded into respective guide holes 38 of thepole piece 22 b. By lowering the lifting table 84, the pole piece 22 bis disposed on the permanent magnet group 20 b, and then fixed onto thepermanent magnet group 20 b by screws (not illustrated). The samearrangement applies to the pole piece 22 a.

[0057] As has been described above, according to the magnetic fieldgenerator 10, the permanent magnet 40 is fixed in advance onto theoutside surface of each annular-projection piece 34, and a predeterminednumber of the annular-projection pieces 34 each mounted with thepermanent magnet 40 are installed onto the base plate 26. Therefore,there is no need for installing the permanent magnets 40 to the annularprojection 32 which has been disposed on the base plate 26.Specifically, the fixation of the permanent magnet 40 becomes easy bydividing the annular projection 32. As a result, assembly of thepole-piece units 11 a, 11 b, and further of the magnetic field generator10 become easy, leading to improved production efficiency even if thereis a need for an intense magnetic field and therefore a R—Fe—B magnet isused as the unit-piece magnet.

[0058] Further, by inserting the guide stake 80 attached on the baseplate 26 into the guide hole 38 of the annular-projection piece 34, theannular-projection piece 34 can be guided to a predetermined place onthe base plate 26. Therefore, positioning of the annular-projectionpiece 34 becomes easy.

[0059] The magnetic field generator 10 as described above and a priorart magnetic field generator were compared in an experiment in whicheach was built to generate a magnetic field of 0.4 T in a space betweentheir respective pole pieces. Results show that the prior art, which didnot use the permanent magnets 40 for reduction of magnetic flux leakage,needed a greater amount of permanent magnet, i.e. 4.9 tons, due togreater magnetic flux leakage. On the other hand, the magnetic fieldgenerator 10, which could reduce the magnetic flux leakage by using thepermanent magnets 40, needed 3.9 tons of permanent magnet. Therefore,according to the magnetic field generator 10, the amount of permanentmagnet to be used can be reduced, and cost can be reduced.

[0060] Next, referring to FIG. 9, a magnetic field generator 100 asanother embodiment of the present invention, to be used for an MRI,comprises a pair of pole-piece units 102 a, 102 b facing each other witha space in between. The pole-piece unit 102 a, 102 b respectivelyinclude plate yokes 104 a, 104 b.

[0061] The plate yokes 104 a, 104 b have respective surfaces facing eachother and provided with permanent magnet groups 106 a, 106 b. Thepermanent magnet groups 106 a, 106 b have respective surfaces facingeach other and provided with pole pieces 108 a, 108 b respectivelyfastened thereto.

[0062] Each of the permanent magnet groups 106 a, 106 b is made bystacking a plurality of unit-piece magnets 110 in three layers. Each ofthe unit-piece magnets 110 is made of a cubic R—Fe—B magnet having aside of 50 mm for example.

[0063] The pole piece 108 a includes a base plate 112 made ofdisc-shaped iron for example, disposed on a main surface of thepermanent magnet group 106 a. The base plate 112 has a main surfaceprovided with a silicon steel plate 114 for preventing generation ofeddy current. The silicon steel plate 114 is fixed with an adhesive ontothe base plate 112.

[0064] The base plate 112 has a circumferential portion formed with anannular projection 116, made of iron for example, for increasingmagnetic field strength around the circumferential portion. The annularprojection 116 is formed essentially in the same way as in the annularprojection 32, and includes a plurality of annular-projection pieces 34as shown in FIG. 3 for example.

[0065] The outside surface of the annular projection 116, i.e. theoutside surface of each annular-projection piece 34, is mounted with apermanent magnet 118 for reduction of magnetic flux leakage. Thepermanent magnet 118 induces magnetic flux to concentrate between thepole pieces 108 a, 108 b, and reduce the magnetic flux leakage. Thepermanent magnet 118 is formed into a generally rectangularparallelepiped block by stacking a plurality of generally rectangularparallelepiped unit-piece magnets each made of a R—Fe—B magnet forexample. Here, attention should be made to a magnetizing direction A1 ofthe permanent magnet 118. As shown in FIG. 9, the permanent magnet 118is formed so that the magnetizing direction A1 makes an acute angle withrespect to the main surface 120 of the permanent magnet 118. Therefore,each of the unit-piece magnet constituting the permanent magnet 118 isformed likewise so that the magnetizing direction thereof makes an acuteangle with respect to a main surface of the unit-piece magnet. Bydisposing the permanent magnet 118 so as to contact a main surface ofthe permanent magnet group 106 a, an outside surface of the base plate112 and the outside surface of the annular projection 116, themagnetizing direction A1 of the permanent magnet 118 can be slanted withrespect to the main surface of the permanent magnet group 106 a. On anoutside surface of the permanent magnet 118, the above describedretainer plate 46 for example is disposed. In this case, the permanentmagnet 118 and the retainer plate 46 are fixed to the annular-projectionpiece 34, i.e. to the annular projection 116, by using a fixing screwfor example. The same applies to the pole piece 108 b.

[0066] The plate yokes 104 a, 104 b are magnetically connected by twocylindrical column yokes 58, in the same manner as in the magnetic fieldgenerator 10 shown in FIG. 1. The plate yokes 104 a, 104 b and thecolumn yokes 58 are mutually connected and fixed together by fixingbolts for example.

[0067] The unit-piece magnets used in the permanent magnet 118 aremanufactured by using a pressing apparatus 200 as shown in FIG. 10 forexample.

[0068] The pressing apparatus 200 comprises a pair of opposed yokes 202(only one yoke is shown in FIG. 10.) The yokes 202 are surrounded bycoils 204 respectively. By applying electric current to the coil 204, amagnetic field is generated.

[0069] A die 206 made of a nonmagnetic or weak-magnetic material isprovided between the pair of yokes 202. The die 206 is formed with aplurality of vertical through holes 208. The through holes 208 eachhaving a rectangular section are formed to slant at a magnetizing angleθ with respect to a magnetic orienting direction B provided by the yokes202.

[0070] Between the pair of yokes 202, a die set 210 is provided,sandwiching the die 206. Further, a base plate 212 is disposed tosandwich the die 206 and the die set 210 perpendicularly with respect tothe magnetic orienting direction B. The die 206, the die set 210 and thebase plate 212 have respective upper surfaces flush among each other.Further, on the base plate 212, a powder feeding box 214 containing apowder is disposed movably toward and from the die 206.

[0071] By using the pressing apparatus 200 as described above, thepowder is magnetically oriented in the through holes 208 and pressed byunillustrated pairs of prismatic upper and lower punches into compacts.The compacts are then sintered to become the unit-piece magnets eachhaving a desired magnetizing angle θ. The method of making the magnet isdisclosed in the U.S. Pat. No. 4,770,723.

[0072] Next, description will cover experiments conducted by using themagnetic field generator 100. The experiments were made under thefollowing conditions: a thickness T1 of the permanent magnet group 106a, 106 b was 150 mm; a total thickness T2 of the base plate 112 and thesilicon steel plate 114 was 65 mm; a height H1 of the annular projection116 was 64 mm, a thickness T3 thereof was 65 mm; a height H2 of thepermanent magnet 118 was 80 mm, a thickness T4 thereof was 120 mm; aninner diameter D1 of the annular projection 116 was 855 mm, an outerdiameter D2 thereof was 983 mm; and a distance d between pole pieces 108a and 108 b was 405 mm. With this arrangement, the uniform space havinga shape of vertically flattened sphere is formed between the pole pieces108 a, 108 b.

[0073] First, in the magnetic field generator 100, the magnetizing angleθ of the permanent magnet 118 was varied, and a horizontal diameter D3of the uniform space and a center magnetic field strength thereof weremeasured.

[0074] From FIG. 11A, it is understood that the uniform space enlargeswhen the magnetizing angle θ of the permanent magnet 118 is increased.Thus, by disposing the permanent magnets 118 with the magnetizing angleθ slanted with respect to the main surfaces of the permanent magnetgroups 106 a, 106 b, greater amount of magnetic flux can be introducedinto an imaging space, and the uniform space can be increased.Therefore, the uniform space can be increased without increasing theamount of permanent magnet 118 to be used and without increasing cost.Further, by adjusting the magnetizing angle θ, a desired uniform spaceand center magnetic field strength can be obtained.

[0075] Next, in the magnetic field generator 100, the diameter of thepole pieces 108 a, 108 b and the thickness of the annular projection 116were varied in accordance with the variation in the magnetizing angle θof the permanent magnet 118 so as to keep constant the horizontaldiameter D3 of the uniform space, and the center magnetic field strengthwas measured. In this experiment, the permanent magnet groups 106 a, 106b were not varied.

[0076] As described above, the uniform space enlarges when themagnetizing angle θ is increased. Therefore, in order to make constantthe diameter D3 of the uniform space, the size of the pole pieces 108 a,108 b should be decreased in accordance with the increase in themagnetizing angle θ of the permanent magnet 118. By using this method,the magnetic field strength in the space can be increased. Thus, thepermanent magnet 118 can be used at an increased efficiency, and in acase when generating a magnetic field of the same strength, the amountof magnet to be used can be decreased to the extent the magnetic fieldstrength is increased. Therefore, it becomes possible to reduce cost andthe weight of magnetic field generator 100.

[0077] From FIG. 11B, it is learned that if the magnetizing angle θ iswithin a range of 0 degree through 60 degrees, the center magnetic fieldstrength can be made not smaller than when the magnetizing angle θ is 0degree. Especially, if the magnetizing angle θ is between 10 degreesthrough 50 degrees, the center magnetic field strength can be notsmaller than 0.42 T, becoming possible to obtain a stronger magneticfield. More preferably, the magnetizing angle θ is between 15 degreesthrough 45 degrees.

[0078] Further, from results of another experiment shown in FIG. 12, ifthe center magnetic field strength and the distance d between the polepieces 108 a, 108 b are kept constant, with the magnetizing angle θbeing set at 30 degrees, it becomes possible to reduce the total weightof the magnet to 2313 kg, which is lighter by 100 kg than when themagnetizing angle θ is 0 degree; and by 610 kg than when the permanentmagnets 118 are not provided. In this experiment, the diameter D3 of theuniform space was 400 mm. Therefore, weights of the plate yokes 104 aand 104 b can be reduced.

[0079] The magnets used as the permanent magnets 118 have themagnetizing direction A1 already slanted. Therefore, by only disposingthe permanent magnets 118 directly on the main surfaces of the permanentmagnet groups 106 a, 106 b, disposition of the permanent magnets 118 forreduction of magnetic flux leakage having a desired magnetizing angle θcan be carried out easily.

[0080] Further, a magnetic field generator 100 a as shown in FIG. 13 maybe used.

[0081] According to the magnetic field generator 100 a, a permanentmagnet 124 is used as the permanent magnet for reduction of magneticflux leakage included in the pole pieces 122 a, 122 b. The permanentmagnet 124 has a magnetizing direction A2 in parallel or vertical toeach surface of the permanent magnet 124. In this case, members 126 and128 each having a trapezoidal section are used in order to dispose thepermanent magnet 124 so that the magnetizing direction A2 of thepermanent magnet 124 is slanted with respect to the corresponding mainsurface of the permanent magnet groups 106 a, 106 b. Other arrangementsare essentially the same as in the magnetic field generator 100 shown inFIG. 9, and therefore will not be repeated here.

[0082] According to the magnetic field generator 100 a, the permanentmagnets 124 themselves are slanted at the desired angle with respect tothe main surfaces of the respective permanent magnet groups 106 a, 106b. Therefore, a permanent magnet magnetized in any direction can be usedas the permanent magnet for reduction of magnetic flux leakage.Particularly, a commonly available permanent magnet magnetized in adirection in parallel or vertical to each surface thereof can be used,making possible to easily obtain the permanent magnets for reduction ofmagnetic flux leakage. Further, the same effect as in the magnetic fieldgenerator 100 can be obtained in the magnetic field generator 100 a.

[0083] It should be noted here that alternatively, the outside surfaceof the annular projection 116 may be slanted, and the permanent magnets118 and 124 for reduction of magnetic flux leakage may be installed tothe slanted outside surface. In this case again, the same effect as inthe magnetic field generator 100 can be obtained.

[0084] Further, the column yoke in the present invention includes aplate-like column yoke such as disclosed in the U.S. patent applicationSer. No. 09/386,146.

[0085] The present invention can also be applied to a magnetic fieldgenerator comprising a single column yoke.

[0086] The present invention being thus far described and illustrated indetail, it is obvious that these description and drawings only representan example of the present invention, and should not be interpreted aslimiting the invention. The spirit and scope of the present invention isonly limited by words used in the accompanied claims.

1-10. (Canceled) 11: A magnetic field generator comprising a pair ofpole-piece units and a column yoke magnetically connecting the pair ofpole-piece units, wherein each of the pole-piece units includes a plateyoke, a permanent magnet group provided on a main surface of the plateyoke, and a pole piece provided on a main surface of the permanentmagnet group, the pole piece including an annular projection, and apermanent magnet for reduction of magnetic flux leakage provided on aflat outside surface of the annular projection. 12: A magnetic fieldgenerator comprising a pair of pole-piece units and a column yokemagnetically connecting the pair of pole-piece units, wherein each ofthe pole-piece units includes a plate yoke, a permanent magnet groupprovided on a main surface of the plate yoke, and a pole piece providedon a main surface of the permanent magnet group, the pole pieceincluding an annular projection, a permanent magnet for reduction ofmagnetic flux leakage provided on an outside surface of the annularprojection, and a retainer for fixing the permanent magnet to theannular projection. 13: An open type magnetic filed generator comprisinga pair of pole-piece units and a column yoke magnetically connecting thepair of pole-piece units, wherein each of the pole-piece units includesa plate yoke, a permanent magnet group provided on a main surface of theplate yoke, and a pole piece provided on a main surface of the permanentmagnet group, the pole piece including an annular projection, and apermanent magnet for reduction of magnetic flux leakage provided on anoutside surface of the annular projection. 14: The generator accordingto one of claims 11 to 13, wherein the annular projection has aplurality of annular-projection pieces. 15: The generator according toone of claims 11 to 13, wherein the permanent magnet for reduction ofmagnetic flux leakage is generally parallelepiped. 16: The generatoraccording to one of claims 11 to 13, wherein the permanent magnet forreduction of magnetic flux leakage is a sintered magnet. 17: Thegenerator according to claim 12, wherein the retainer includes aretainer plate and a screw. 18: A Pole-piece unit comprising: a plateyoke; a permanent magnet group provided on a main surface of the plateyoke; and a pole piece provided on a main surface of the permanentmagnet group; wherein the pole piece includes an annular projectionhaving a plurality of annular-projection pieces, and a permanent magnetfor reduction of magnetic flux leakage provided on an outside surface ofeach of the annular-projection pieces. 19: A method for assembling apole-piece unit including an annular-projection piece having a flatoutside surface, wherein a permanent magnet is slid on the outsidesurface for fixation of the permanent magnet to a desired position onthe annular-projection piece.